A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning structure, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.
The term “patterning structure” used herein should be broadly interpreted as referring to a structure that can be used to impart a beam of radiation (e.g. a projection beam) with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the beam may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.
A patterning structure may be transmissive or reflective. Examples of patterning structures include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned.
The support structure supports, i.e. bares the weight of, the patterning structure. It holds the patterning structure in a way depending on the orientation of the patterning structure, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning structure is held in a vacuum environment. The support can use mechanical clamping, vacuum, or other clamping techniques, for example electrostatic clamping under vacuum conditions. The support structure may be, for example, a frame or a table, which may be fixed or movable as required and which may ensure that the patterning structure is at a desired position, for example, with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning structure”.
The term “projection system” used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for example for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term “lens” herein may be considered as synonymous with the more general term “projection system”.
The illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components configured to direct, shape, or control the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”.
The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory tasks may be carried out on one or more tables while one or more other tables are being used for exposure.
The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the first element of the projection system. Immersion techniques may be used to increase the numerical aperture of projection systems.
The continuous trend towards smaller design features and higher device densities requires high resolution lithography. In order to meet the requirements, it may be desirable to control the lithographic process in as many details as possible. Two of the most important process parameters that may need accurate monitoring and control are dose and focus. Generally the critical dimension (CD) variations are measured to monitor and control these parameters. However, it may be difficult to discriminate between dose and focus data when measuring CD-variations.
In general, special or multiple features are used in combination with special or time consuming metrology. The focus can for instance be determined by a phase shift focus monitor. The focus error results in an overlay error that can easily be detected with an overlay readout tool. In a second technique, the monitoring of the focus is achieved by using the concept of line-end shortening. However, with this technique the sign of defocus may be extremely difficult to determine. Additionally, most present-day techniques are only applicable on test structures.
The need to monitor the quality of the pattern that is exposed by a lithographic apparatus calls for a fast and reliable technique, which can be used at many locations, for example within a chip area or in a scribe line, on all kinds of substrates to be exposed, like test or product wafers. An optical metrology technique, called scatterometry, can meet these requirements to a certain extent. The terms “optical” and “light” used herein encompass all types of electromagnetic radiation, including light with a wavelength of 400-1500 nm, ultraviolet (UV) radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultraviolet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.
In scatterometry, a light beam is directed towards a target, generally a specially designed structure like a diffraction grating. Then, the target reflects, refracts and/or diffracts the light. Finally the light from the target can be detected by a detector including a suitable sensor. The detection by the detector can be in reflection or in transmission, measuring the diffracted and/or non-diffracted light. For the incoming light, i.e. the light directed at the target, one or more sets of properties can be varied simultaneously. The terms “scatterometry” and “scatterometer” used herein encompass all types of measurement techniques and tools in which light is generated and analyzed after interaction with a target. The term “scatterometer” thus includes, for example, an ellipsometer and a scanning electron microscope (SEM). The term “spectrum” used herein encompasses all types of formats in which the light after interaction with the target can be detected. It thus includes images created by scattered electrons in a SEM.
Scatterometry is conventionally used to determine the values of process parameters, like focus and dose. Generally, however, several assumptions are made regarding the relationship between process parameters and scatterometry measurement parameters. Examples of such assumed relationships are a linear relationship between focus and side wall angle (the slope at the side of a line-shaped structure) and a linear relationship between dose and mid-CD (the width of a line-shaped structure at half its height). In reality, there may be no unique relationship between one single scatterometry measurement parameter and a process parameter like focus or dose. There may be, for example, additional effects, besides focus, that contribute to the characteristics of a side wall angle. By the aforementioned assumption, these effects would then be abusively interpreted as focus.
The detected spectrum (or, in the case where particle beams are used, the detected signal may be an image rather than a spectrum) is analyzed by comparing it with data, stored in a library. A so-called “best match” between the detected spectra and the spectra in the library determines the parameter values that best describe the target structure. For lithographic purposes, the identified parameter values, i.e. focus and dose, can be applied to increase the performance of a lithographic apparatus. The quality of lithographic process parameter control and monitoring may strongly depend on the quality of the library. A library is generally filled with theoretical spectra constructed by calculating values for different scatterometry measurement parameters such as grating parameters like grating height, line width and side wall angle, and different substrate parameters, such as material properties and properties related to layers in the substrate processed earlier. It can easily be understood that the creation of an extremely reliable library can be time-consuming and highly complex, especially when the properties of the substrates to be exposed change regularly.
Furthermore, scatterometry measurement parameters, like the thickness of the underlying layers and the optical constants of the used materials, may be extremely difficult to determine in a production situation. The use of empirical data, i.e. experimentally obtained data, has been suggested. (See for instance Allgair et al., Yield Management Solutions, Summer 2002, pp 8-13). In such a case, the empirical library is then generated from a substrate with a number of structures processed by a varying set of process parameters covering the process space to be controlled. However, as mentioned in this reference, the characterization of these structures is non-trivial, due to the required level of control over the process parameters and an important influence of noise introduced by ‘natural variation’, i.e. not deliberately induced variations.